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My house has been fitted with water traps in toilet and bathrooms drains which is quite good, but I can see that my kitchen sink, washing machine outlet pipe and regular wash basin don't have water traps.

Shouldn't we have water traps to avoid gas or cockroach from entering the building through drainage?

My plumber argued that keeping the extra water traps in kitchen sink and other places will have dirty water and this can be smelly but I'm not buying his argument.

Additional info: The town that I'm living in doesn't have integrated sewer or grey water collection system.

It's just that I have six dry wells for grey water collection around my home. All 6 dry wells have a 1 foot high air vent. I'm just worried that toxic gas or mosquito or cockroach or some other rodents might enter through the plumbing line.

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If your drainage system isn't self-contained--if it has exterior downstream venting to the atmosphere--then local traps aren't beneficial in the sense that they block sewer gas. That would already be let free outside. There's no pressure pushing gas back into the home. Your plumber is correct that they're little more than repositories for waste material.

However, they may offer some protection against pest ingress (though I suspect cockroaches in particular wouldn't be dissuaded from entry by a little puddle).

Therefore the answer probably comes down to the nuances of your situation and your priorities with what could be considered a less-than-ideal wastewater system.

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  • Would it make sense to just install a P trap on the outlet of the building. There is one outlet for all the bathroom and there's one outlet for all the kitchen. So just installing two traps would be suffice for entire building ?
    – Amogam
    Oct 10, 2022 at 19:54
  • Your answer makes more sense to me.There's no exterior pressure to push the air back into the home and the water is not going to be demotivate the pests
    – Amogam
    Apr 9 at 19:31
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    Traps need to be very close to the source of the drainage and have local venting or they can be forced empty by water from above or below. It may work, but it probably wouldn't.
    – isherwood
    Apr 9 at 21:20
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    Yes and it also can becomes a place where all the wastes gets stored and clog the line and it might need manual snaking out
    – Amogam
    Apr 10 at 16:25
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Not only you should have water traps (called P-trap)

But you should also have air vents pipes up to the roof, to assist water drain.

Some appliances like toilet come with water trap build in, for obvious reason.

Every other water drain should have P-trap.

The argument of having the water residue in P-trap will lead to smells, is rediculous.

However the P-trap can dry out (no water) if not used for very long time, in which case you will get the sewer smell.

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  • Not all traps are P-traps. That's just a particular configuration.
    – isherwood
    Oct 7, 2022 at 21:26
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    My 6 dry wells have , 1 feet high air vent for each. If the air vent is extended upto the roof , would it help in draining the water fast ?
    – Amogam
    Oct 8, 2022 at 19:19
  • Would it make sense to just install a P trap on the outlet of the building. There is one outlet for all the bathroom and there's one outlet for all the kitchen. So just installing two traps would be suffice for entire building ?
    – Amogam
    Oct 9, 2022 at 8:29

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